Such tensioning is necessary, firstly to enable the projection surface to be rolled up around the roll-up tube, and secondly to ensure the projection surface is plane when it is unrolled, which is a necessary condition for obtaining good quality projection.
This condition of planeness is achieved in part only by such a solution, since the sole force exerted on the projection surface is the force exerted by the weight in the vertical direction. This generally results in the unrolled projection surface being approximately plane, while presenting, in detail, two types of deformation: either a swelling in a low middle zone, or edges deformed into circular arcs.
Although such deformations are considered to be acceptable in numerous applications, they are unacceptable in other applications, in particular for home use, where a high level of image quality is required.
Devices suitable for exerting lateral tension on roll-up projection surfaces are known. In general, they comprise fastener tabs disposed at a predetermined pitch along the side edges of the projection surface. These tabs are connected to two tensioned strings on respective sides of the screen. The strings are themselves tensioned by vertical traction using a horizontal bar that also acts as a weight for the projection surface, so as to provide the necessary vertical tension.
The drawbacks of such a device are known. Such lateral tensioning devices are expensive to fabricate, often requiring manual adjustment. Furthermore they are unattractive, often leading to the side edges of the screen being subdivided into circular arcs. In addition, they require a roll-up tube that is considerably wider than the usable projection surface. Finally, they prevent or make difficult any modification to the adjustment at the end of the rolling stroke, determining the height of the bottom edge of the projection surface, and thus the vertical position of said surface.
Another defect also often affects the quality of the image projected onto a roll-up screen. Since the environment in which the screen is to be found can vary, it often happens that a pale surface lies behind the screen. This surface reflects some of the light coming from the projected image by virtue of transparency of the screen. This is particularly visible when the screen is perforated or woven so as to be permeable to sound. The light as reflected in this way returns by transparency and becomes superposed on the projected image, adding interference and thus making it less sharp.
To avoid that phenomenon, it is common practice to use a coating of black color on the rear face of the screen, serving to absorb light reflected by a wall situated behind the screen. Nevertheless, such a coating is particularly difficult to implement and expensive on a screen that is perforated or woven. It is difficult to paint only one face of such a porous surface black without changing the appearance of its other face, where said other face must necessarily remain white or pale gray. A known solution consists in rolling a second layer up simultaneously with the projection surface, the second layer being black or very dark gray and constituting an occultation layer, generally made of a material that is similar to the projection surface and presenting equivalent permeability to soundwaves. This layer is located behind the projection surface when the screen is unrolled. The extra cost is significant.
A characteristic that is desirable for screens that are for use with high quality video projection is to frame the projection surface with a black border, thereby reinforcing the perception of image contrast. Making such a black border, generally by painting the margins of the projection surface, has the drawback of involving extra cost.